Port Levy Rainfall

Introduction

We've long thought that we get more rain in Port Levy than than they do at Lyttelton
and now that we have a PWS at Port Levy, we can find out for sure by comparing measurements.

Nearby AWSs

There are four nearby AWSs, as shown in the map to the left, where the AWSs are marked with a red blob.

The AWSs have data extending back many decades, whereas we have only at short record at the Port Levy PWS (25-Jun-2010 onwards),
so we will compare rainfall events rather than the usual comparison of monthly totals.

Nearby PWSs

There are five nearby PWSs, as shown on the map to the left.

These PWSs are maintained by amateurs who kindly post their data to the
Weather Underground
site.

The webpages on Weather Undeground associated with each of these PWSs are:

Rainfall Events During August 2010

During August 2010, there were two significant rainfall events from the southeast which resulted in the monthly mean rainfall at most sites
being substantially more than the long-term median for August.

In the following, we compare the rainfall during these two events for the Lyttelton and Le Bons Bay AWSs and
all the PWSs.

In comparing daily rainfall between the PWS and AWSs there are differences that are caused by the different definitions of a day:

For the PWS: a day goes from midnight to midnight

For an AWS: a day goes from 0900 to 0900 - this is traditional and goes back to when measurements were done manually at 9 am every day

Thus, if it rains between midnight and 0900, that rain will be attributed to the previous day in the AWSs,
but to the present day in the PWS.

08-Aug-2010

In this event, the rainfall at Port Levy was much larger than that at Lyttelton, but about the same as the rainfall at Le Bons Bay.

For the PWSs, the Port Levy rainfall was about the average of all the other PWSs.

The Diamond Harbour PWS was lower than the rest and similar to the Lyttelton AWS.
In fact these two gauges have similar aspects, facing east to the harbour entrance,
albeit on either side of the harbour.
The Lyttelton PWS, which is up behind Lyttelton, had much more rainfall than either of these.

18/19-Aug-2010

In this event, the Port Levy PWS had slightly more rainfall than Lyttelton, but a lot more than Le Bons Bay

The rainfall at the PWSs was about the same, except for Diamond Harbour, which was much less.

Discussion

There are lots of reasons why rainfall measured at different stations differs:

Rainfall patterns are highly variable as a result of differing topography;

The position of the raingauge is crucial — e.g., is it sitting under a tree?

The design of the collecting dish of the raingauge is important — does rain splash out?

The storm direction is important — at Port Levy, we get more rain in southerlies than in easterlies.

In the two southeasterly storms in August 2010, the rainfall at the Lyttelton PWS was substantially greater than the rainfall at the AWS.

Their locations are shown to the left.

Evidently, the location of the AWS on Z-Berth means that it is more sheltered from storms like these.

Conclusion

The location of the Lyttelton AWS at Z-Berth means that rainfall measured there is not a good indicator of rainfall elsewhere on the Peninsula